Study On Mechanical Properties And Structure Of Mesophase Pitch-based Carbon Fibers By Monofilament Tensile Test

The mechanieal properties of carbon fibers are controlled by its structure. At present, the strength and modulus of carbon fibers are much lower than their theoretical values. The strength of pitch-based carbon fiber, are generally lower than PAN-based carbon fiber. To investigate fracture morphology and their failure mechanism under tension, the fracture surfaces obtained from the monofilament tensile testing of mesophase pitch-based carbon fibers were examined by scanning electron microscope, along with their tensile performance.Differing significantly from polyacrylonitrile-based carbon fibers and some ceramic fibers, mesophase pitch-based carbon fibers exhibited a highly variable fracture morphology due to their inherent structure of long range crystallites and defects. Five types of fracture surfaces have been identified for a fiber sample with a radial texture, and relationships were found between the fracture morphology and tensile strength, with planar surfaces having the lowest strength and extensively fragmented surfaces giving the highest tensile strength.Three types of fracture surfaces have been identified for a fiber sample with a rondom structure, and relationships were found between the fracture morphology and tensile strength, with planar surfaces having the lowest strength and mountain-valley surfaces giving the highest tensile strength. Differing from radial structure of mesophase pitch-based carbon fiber, a new fracture morphology-grooved fracture morphology is found. Probably due to a relatively low carbonization temperature applied during heat treatment, extensively fragmented surfaces were not observed in the random structure of P2.As can be seen by the study of pitch-based carbon fiber fracture surface, carbon fiber with relative planar surfaces exhibits lowest strength. As their strength increases, the fracture surface becomes uneven and rough.A non-contact strain measuring system was explored using video extensometry. For mesophase-based carbon fibers, the strain levels are very low, so although the validity of video extensometry is proved by experiment data, the accuracy is still to be imporved using CMOS camera with higher resolution while maintain higher frame rate.